1. Field of the Invention
The present invention relates to hydraulic systems for an off-road vehicle, such as agricultural and construction equipment, and in particular to hydraulic systems on a tractor which can be configured to control actuators on different types of work implements removably connected to the off-road vehicle.
2. Description of the Related Art
With reference to FIG. 1, an agricultural tractor 10, of conventional construction, includes a main body carried on front steerable wheels (not shown) and rear tractor wheels 16. The rear portion of the tractor is illustrated with one wheel removed and portions of the tractor body broken away to reveal components of a standard three-point hitch 15. An agricultural implement 12, in this case a cultivator, is mounted to the three-point hitch 15 of the tractor 10.
The three-point hitch 15 comprises of a pair of trailing lower draft links 18 and an upper compression link 20. The draft links 18 are raised an lowered by an actuator comprising a single-acting rockshaft hydraulic cylinder 24 having a rod 26 coupled with a transversely extending rockshaft-lever assembly 30. The rockshaft-lever assembly 30 is pivotally connected to respective draft links 18 by drop links 28. When the rockshaft cylinder 24 is driven by pressurized hydraulic fluid, the rod 26 extends thereby raising the draft links 18. As hydraulic fluid is released from the rockshaft hydraulic cylinder 24 and the rod 26 is free to move, the draft links 18 drop and rise as the implement follows the ground contour. An upstanding coupler 32, at the rearward ends of the draft and compression links 18 and 20, is provided for connecting different implements to the three-point hitch 15.
The illustrated implement 12 includes a forwardly projecting mast structure 34 which engages the coupler 36 of the tractor 10. A toolbar 38 connected to the mast structure 34 supports a plurality of rearwardly projecting earth-working tools 40, such as cultivator shovels. A rearwardly extending lift assist frame 42 is rigidly connected to the toolbar 38 and has a trailing end to which a pair of parallel suspension links 44 are pivotally connected. The suspension links 44 in turn are pivotally attached to a wheel support 46 which carries a lift assist wheel 48. A double-acting hydraulic cylinder 49 is connected between the toolbar 38 and lift assist frame 42 lower the lift assist wheel 48 to the ground for transporting the implement 12 on a road or raise the lift assist wheel for plowing. The double-acting hydraulic cylinder 49 is connected by a pair of hoses to a hydraulic coupling at the rear of the tractor.
FIG. 2 illustrates the hydraulic system of the tractor 10 for powering the cylinders 24 and 49. Specifically, the hydraulic system 60 has a tank 62 containing hydraulic fluid. A pump 64 driven by the engine of the tractor fluid, draws hydraulic fluid from the tank 62 and sends the fluid under pressure into a supply line 66. A return line 68 is provided to bring the fluid back to the tank after passing through the actuators 24 and 49. A first three-way, three-position hydraulic valve 70 is coupled to the supply and return lines 66 and 68. The position of the first control valve 70 is governed by a mechanical lever 72 located on the tractor near the operators seat. The first control valve selectively couples either the supply line or the return line 66 or 68 to the single-acting rockshaft hydraulic cylinder 24. At one position of the first control valve 70, pressurized fluid from the supply line 66 is sent to the head chamber of the rock shaft hydraulic cylinder 24 to extend the piston. In another position of the first control valve, the head chamber of the rock shaft hydraulic cylinder 24 is connected to the tank return line 68 so that gravity operating on the hitch 15 forces the fluid from the cylinder to the tank 62 allowing the hitch and the implement 12 to lower. In the center, neutral position of the first control valve 70, the rockshaft hydraulic cylinder 24 is disconnected from both supply and the return lines 66 and 68.
A second control valve 74 is connected by a quick connect coupling 75 and hoses to the lift assist cylinder 49 on the implement 12. The second control valve 74 is a four-way, three-position valve that is operated by manual lever 76. In one extreme position of the second control valve 74, fluid is sent from the supply line 66 to the head chamber of the lift assist cylinder 49 and the fluid draining from the rod chamber is sent to the tank return line 68. In the other extreme valve position, the flow is reversed, i.e. pressurized fluid from the supply line 66 is fed to the rod chamber of the lift assist cylinder 49 and fluid is drained from the head chamber to the tank return line 68. This second control valve 74 also has a center, neutral position. A third control valve 78 on the tractor a four-way, three-position valve operated by lever 80. This control valve 78 feeds another quick connect coupling 82 which is not connected to the implement 12 in FIG. 1, as that implement does not have additional hydraulic actuators.
Some implements have a greater number of hydraulic actuators than implement 12 in FIG. 1, and thus, utilize the additional control capability provided by the third control valve 78. For example, the implement may have a single-acting hydraulic cylinder 84 which operates an implement component. In this case, only one hose is required for connection to the third control valve 78, and thus only one port of the quick connect coupling 82 is utilized. Although the second port of quick connect coupling 82 is unused in this case, another hydraulic actuator can not be connected to that unused port unless that additional actuator operates in a reverse tandem manner to hydraulic cylinder 84. Therefore, unless both of these additional actuators are to be operated in unison and in opposite directions (one actuator receives hydraulic fluid from supply line 66, while hydraulic fluid is being drained to the tank from the other actuator), it is impractical to connect another actuator to this unutilized port of quick connect coupling 82. Therefore conventional tractor hydraulic systems typically provide separate four-way control valves for each independently controlled actuator on the implement. This is may require a relatively large number of control valves and port pairs.
Agricultural tractors traditionally utilized manually operated control valves as shown in FIG. 2, which were mechanically operated by a levers manipulated by the farmer driving the tractor. There is a present trend away from hydromechanical controls toward electrohydraulic systems in which each valve is operated by a solenoid or other type of electrically powered mechanism. This simplifies the physical layout of the equipment, as all the valves to not have to be located relatively near the operator lever. Instead, an input device such as a joystick, located at the driver's position, is connected by electrical wires to control circuits that operate the solenoid valves situated anywhere on the tractor. Electrohydraulic controls also enable computerized control of the hydraulic system, which can provide many functions and features which are not practical with mechanical controls. For example, a single joystick motion may cause fluid at different flow rates to be supplied to several actuators.